1. Field of the Invention
The present invention relates to a pneumatic tire, where a raised bottom portion is provided in a groove created on the tread surface.
2. Description of the Related Art
In general, a variety of tread patterns are formed on the tread surface of pneumatic tires, in accordance with required tire functions and conditions for use. In particular, a tread pattern which is referred to as a block type has a number of blocks separated by grooves, and it is known that the pattern has excellent performance on snowy roads, excellent water dispersing performance and excellent traction. There is a problem, however, where the blocks separated by grooves have a relatively low rigidity and irregular wear easily occurs on the tread surface. This problem is shared by other pneumatic tires where block type patterns are formed, and in particular, is significant in studless tires where a great number of sipes are created in the blocks.
For this reason, pneumatic tires where raised bottom portions are provided to the grooves on the tread surface have been proposed in the following Specification of Japanese Patent 3116243, Japanese Unexamined Patent Publication 62-4608(1987) and Japanese Unexamined Patent Publication 63-125410(1988). Those pneumatic tires are provided with raised bottom portions in the corners where the rigidity is relatively low in the blocks, and thereby, the rigidity of the blocks can be increased, restricting the occurrence of irregular wear. In the above described configurations, however, closed spaces surrounded by the raised bottom portions are created so as to have a length that corresponds to the pitch of the pattern on the tread surface. As a result of this, when the tread surface is worn to the point where the raised bottom portions make contact with the ground, in some cases, the frequency band of the pumping sound which is emitted by the compression and release of air within a closed space that faces the road surface becomes close to pitch noise, and a problem arises where the pumping sound resonates with the pitch noise, increasing the level of noise.
Here, though the below described Specification of Japanese Patent 3511413 also discloses a pneumatic tire where raised bottom portions (protrusions) are provided in the grooves on the tread surface, these raised bottom portions give a warning that the tread surface is in the last phase of wearing by means of a pumping sound, and the rigidity of the blocks is not sufficiently increased. In addition, the raised bottom patterns are provided on both sides of the grooves, and thereby, closed spaces are created so as to have a length that corresponds to the pitch of the pattern on the tread surface, and therefore, in some cases, a pumping sound resonates with pitch noise in the same manner as described above, increasing the level of noise.